In the past, easy open can lids enabling manual opening of part of the container lid have been made by placing the lid body on a flat lower mold half and pressing a sharp blade having the required contour shape from the top surface so that the cutting edge bites into the lid body whereby the V-shaped cross-section tear-along groove shown in FIG. 6 is formed and the shape of the opening piece is obtained. With this method, however, since the cutting edge was sharp, the coating film provided in advance for prevention of corrosion would be broken, necessitating repair coating later in the case of a steel lid and thereby resulting in higher manufacturing costs.
One method which has been recently used to prevent corrosion in place of the conventional lacquer coating has been to use a thermoplastic resin film at some locations. A thermoplastic resin film has a relatively superior drawability, and therefore, possibly would enable processing of an opening piece without breaking it depending on the processing method used. When forming a V-shaped cross-section tear-along groove by the above sharp blade, however, it is not possible to completely avoid breakage of the resin film.
The easy open lid (or easy open end) used for beverage cans, general food cans, and the like may be roughly classified into two types: ones provided with a tab using the lever principle and ones without a tab which allow people to directly push and open the lid by finger pressure. The ones provided with tabs further may be broken down into ones where the opening piece comprising part or all of the container lid is torn off by the tab to separate from the can body, i.e., the tear off types, and ones where it is left attached to the can body, i.e., the stay-on tab types. Both types use a coated aluminum sheet or sheet steel as a material, punch them out into the basic lid shape, place them on a flat bottom mold half, press an upper mold half having a sharp edge cross-section scoring blade projecting from it in the shape of the contour of the opening, and form an opening groove of the shape of the opening in the material. To facilitate the opening of the can, it is necessary to press the scoring blade to a depth of the opening guide groove of 1/2 to 2/3 of the thickness of the sheet before processing. If the depth of the opening guide groove is too shallow, the openability will be poor, while if too deep, the strength will be insufficient and the can will open upon even a small impact from the outside.
When the materials of easy open can lids are made extremely thin due to demands for easy opening etc., a considerable precision has been sought in the scoring tools as well, resulting in an extremely short life of the tools. Due to this problem, as described in Japanese Unexamined Patent Publication (Kokai) No. 55-70434 and Japanese Unexamined Patent Publication (Kokai) No. 57-175034, measures have been devised for prolonging the tool life by forming a thin upward-facing connecting piece between the opening piece and its surroundings and then pressing down the opening piece to make the connecting piece bend in the middle and form the opening guide groove and thereby form a tear-off type opening piece for the can.
Further, due to the fact that, with the conventional processing of an opening guide groove, defects occur in an organic film layer and the metal surface becomes exposed, repair coating has been applied to ensure resistance to corrosion from the contents and prevent outside rust, but this repair coating also requires a complicated long baking process similar to that of the main coating work and further results in the discharge of a large amount of solvents contained in the coating, and therefore, due to environmental considerations, the discharged solvent must be incinerated in a special incinerator. In addition, since the heating during the baking of the coating and the incineration of the solvent result in the discharge of carbon dioxide, there were concerns of the load on the global environment.
Recently, as proposed in Japanese Unexamined Patent Publication (Kokai) No. 6-115548, Japanese Unexamined Patent Publication (Kokai) No. 6-115546, and Japanese Unexamined Patent Publication (Kokai) No. 6-122438, to eliminate the need for repair coating and to solve the above problems, a technique has been developed for producing an easy open can lid comprising of pressing a plastic laminated metal sheet by the shoulder radius of an upper and lower mold half to form the opening guide groove, but the openability of the can is not necessary sufficient.
Accordingly, we disclosed, in Japanese Unexamined Patent Publication (Kokai) No. 6-170472, conditions for producing a lid superior in openability limiting the shoulder radius of the upper and lower mold halves for forming the tear-along groove and the residual thickness of the tear-along groove portion after processing.
Further, the specification alluded to the fact that push-back processing reducing the step difference between the lid body and the opening portion was advantageous to the can openability. Just push-back processing alone, however, is not sufficient for achieving an improved can openability. In some cases, it causes breakage of the resin film. It has become clear that push-back processing suited to the state of processing of the guide groove of the opening portion is necessary. In particular, it has become clear that the relationship of the clearance between the die and the punch at the time of processing the opening portion, something not alluded to at all in Japanese Unexamined Patent Publication (Kokai) No. 6-170472, is important.
Further, it has become clear that it is possible to improve the can openability without breaking the surface resin film by the use of push-back processing which bends the metal into an S-shape after the pressing.